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Artificial Intelligence and Soft Computing: Behavioral ... - Arteimi.info

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8. p \/ (q Λ ¬ q) ⇔ p<br />

9. p Λ (q \/ ¬ q) ⇔ p<br />

10. p → q ⇔ ¬ p \/ q<br />

11. ¬ (p→ q) ⇔ p Λ ¬q<br />

12. p ↔ q ⇔ (p → q) Λ (q → p)<br />

13. p ↔q ⇔ (p Λ q) \/ (¬p Λ ¬q)<br />

14. p→ (q→ r) ⇔ (p Λ q) → r<br />

15. p→ q ⇔ ¬ q → ¬p (contraposition theorem)<br />

The syntactic approach for theorem proving can be done in two ways,<br />

namely, i) by the method of substitution <strong>and</strong> ii) by Wang’s algorithm.<br />

5.4.2.1 Method of Substitution<br />

By this method, left-h<strong>and</strong> side (or right-h<strong>and</strong> side) of the statement to be<br />

proved is chosen <strong>and</strong> the st<strong>and</strong>ard formulas, presented above, are applied<br />

selectively to prove the other side of the statement.<br />

Example 5. 3: Prove the contraposition theorem.<br />

The contraposition theorem can be stated as follows. When p <strong>and</strong> q are two<br />

propositions, the theorem takes the form of p→ q ⇔ ¬ q → ¬ p.<br />

Now, L.H.S. = p→ q<br />

⇒ ¬ p V q [by (10)]<br />

⇒ q V ¬ p<br />

⇒ ¬ (¬ q) V ¬ p<br />

⇒ ¬ q → ¬ p = R.H.S. <br />

Analogously, starting with the R.H.S, we can easily reach the L.H.S. Hence,<br />

the theorem bi-directionally holds good.<br />

Example 5.4: Prove theorem (14) by method of substitution.<br />

Proof: L.H.S. = p→ (q→ r)

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